Evaluation of the electrochemical active surface area for carbon felt and nanostructured Ni coatings as electrocatalysts for hydrogen evolution reaction Текст научной статьи по специальности «Нанотехнологии»

NANOSYSTEMS: Dmitriev D.S., Tenevich M.I. Nanosystems:

PHYSICS, CHEMISTRY, MATHEMATICS Phys. Chem. Math., 2023,14 (5), 590-600.

http://nanojournal.ifmo.ru

Original article DOI 10.17586/2220-8054-2023-14-5-590-600

Evaluation of the electrochemical active surface area for carbon felt and nanostruc-tured Ni coatings as electrocatalysts for hydrogen evolution reaction

Dmitry S. Dmitriev, Maksim I. Tenevich loffe Institute, St. Petersburg, Russia

Corresponding author: Dmitry S. Dmitriev, elchemorg@gmail.com

PACS 82.45.Rr

Abstract This study is devoted to the evaluation of electrochemical active surface area (ECSA) for carbon felt used in various fields of electrochemical technology. For the evaluation, we used techniques based on Faraday's law, the Randles-Sevcik equation and the calculation of the electric double layer capacitance in the electrolyte with different pH value. The measurement results are consistent with each other and for neutral, acidic and alkaline medium, the ECSA value are 20 - 30, 30 - 40 and 50 - 90 cm2 per 1 cm2 of geometric surface, respectively. Based on the results, the synthesis of nanostructured nickel coatings on carbon felt with prior electrochemical activation was performed. The pre-treatment in 1M KOH vs 1 M Na2SO4 reduces the crystallite size from 26 to 15 nm and increases the ECSA from 133 to 700 cm2 per 1 cm2 of geometric surface. These changes cause an improvement in other electrocatalytic features for hydrogen evolution reaction. Keywords carbon felt, electrochemical surface area, electrodeposition, voltammetry, double layer capacitance, Randles-Sevcik equation, nickel coating, hydrogen evolution reaction

Acknowledgements The authors express their gratitude to the Engineering Center of the St. Petersburg State Institute of Technology for the prompt possibility of performing studies on a scanning electron microscope For citation Dmitriev D.S., Tenevich M.I. Evaluation of the electrochemical active surface area for carbon felt and nanostructured Nicoatings as electrocatalysts for hydrogen evolution reaction. Nanosystems: Phys. Chem. Math., 2023,14 (5), 590-600.

1. Introduction

The development of new electrode materials is carried out in all fields of electrochemical technology from electrocatalysts of water splitting reactions to chemical current sources and supercapacitors. Currently, the use of porous carbon materials (fibers, felt) as a substrate is popular. Carbon felt is a polyacrylonitrile (PAN) or viscose fiber that has been needle-punched to form a structure and carbonized at 1200 - 1600 °C or graphitized at 2000 - 2600 °C [1]. Carbon felt is widely used in redox-flow batteries [2-7], lithium-, sodium- and zinc-ion batteries [8-15], as well as in supercapacitors [16-21] and alternative energy storage and generation technologies [22-25]. Mingxing Wu et al. have shown that the inclusion of cobalt particles in carbon fibers [13] increases the efficiency of the zinc-air battery, and boron doping [16] improves their capacitive properties when used as supercapacitor electrodes. In addition, carbon fibers are used in the synthesis of electrocatalysts for the reaction of hydrogen, oxygen production and the Fenton process [26-34]. Paper [30] reports on the synthesis of rhenium disulfide nanosheets on the carbon fiber surface as an effective electrocatalyst of the hydrogen evolution reaction (HER) with overpotential values of 69 mV@ 10 mAcm-2 and Tafel slope of 90 mVdec-1.

Unlike a metal substrate, carbon materials are economically and environmentally beneficial, more resource-intensive and can be removed thermally in the next, i.e. they are able to perform the role of templates [35-39]. This makes it possible to significantly increase the electrochemically active surface area of electrode materials, which has a positive effect on their functional features. Pawel Jakobczyk et al. [38] have shown that thermal and chemical modification of the carbon felt surface improves its catalytic properties in the acetaminophen degradation reaction. The authors of [26] declare the successful use of modified carbon felt as a cathode in the Fenton process for the removal of tetracycline. Modification was carried out together by chemical and thermal methods. This allowed one to reduce the relative concentration of tetracycline during the Fenton process by two times. In paper [36], we previously showed that the use of carbon felt as a template makes it possible to obtain copper microtubes with a lower overpotential of HER.

However, the authors using carbon felt in their research are often limited to their geometric (visible) surface area (Sgeo, cm2) or report the surface area of the final material after activation or modification of the fiber. In the datasheet of carbon felt from various manufacturers, approximate values of the surface area are given, usually estimated by the method of low-temperature adsorption calculated using the Brunauer-Emmett-Teller (BET) method [40-48]. These values do not always correspond to the electrochemical active surface area obtained by electrochemical methods [1,49-51]. The value of the specific surface area (Am, m2g-1) obtained by the BET method varies from 0.12 to 8 m2g-1. The most commonly mentioned value is 1.1 - 1.3 m2g-1. The results of ECSA obtained by electrochemical methods (from the

© Dmitriev D.S., Tenevich M.I., 2023

Randles-Sevcik and Cottrell equations) are also different and are given by the authors without taking into account mass, geometric surface or volume. On average, for a sample with a visible surface of 10 cm2, the ECSA value is 40 - 50 cm2.

In this paper, we have tried to evaluate the ECSA by three different electrochemical techniques: 1) on the basis of Faraday's law, measuring the thickness of nickel coatings obtained during coulostatic and galvanostatic modes of electroplating; 2) from calculating the value of electric double layer capacitance (EDLC), as the most common method due to its simplicity and clarity of the values obtained; and 3) from the Randles-Sevcik equation for single-electron redox process involving potassium ferrocyanide.

Based on the results, the synthesis of nanostructured nickel coatings on carbon felt with electrochemical surface pre-treatment in a neutral and alkaline medium was carried out. The electrocatalytic activity of these coatings was evaluated and the ECSA value was measured.

2. Experimental detail

2.1. Materials

The following materials and reagents were used in the study:

Carbon Felt (CF) (Beijing Great Wall Co., Ltd.)), Nickel sulfate (NiSO4 x 7H2O), Sodium sulfate (Na2SO4 x 10H2O), Boric acid (H3BO3), Potassium chloride (KCl), Sulfuric acid (H2SO4), Potassium hydroxide (KOH), Potassium ferrocyanide (K4Fe(CN)6 x 3H2O) from Nevareaktiv.

Platinum foil (Pt, 99.999 %), Copper wire (Cu, 99.999 %), Nickel plates (Ni, 99.99 %), Distilled water (ft = 18MOhm cm) were also used in this work.

2.2. Experimental and measurement conditions

2.2.1. Nickel electroplating. Carbon felt (1.25 x 0.5 x 0.5 cm) with copper wire (d = 0.4 mm) as a collector was used for nickel electroplating. The geometric surface area was Sgeo = 3 cm2. Electroplating was carried out in Watts electrolyte (Table 1) at the room temperature with a magnetic stirrer. Nickel plates with a working surface area of Sa = 10 cm2 were used as anodes. The experiment was carried out in two series: with a fixed current value (30 mA) and with a constant amount of electricity (30 mA h). In the both series, the process was performed for 0.5, 1, 2 hours.

TABLE 1. Electrolyte composition and nickel electroplating mode

Component Concentration g/L

N1SO4 x 7H2O 75

Na2SÜ4 x IOH2O 50

H3BO3 25

KCl 10

pH = 5.6 ± 0.1; T = 25 °C; 1st series: I = 30 mA=const; t = 0.5, 1, 2 h; 2nd series: Q = It = 30 mA h(15mA 2h, 30mA1 h, 60mA 0.5 h)

Before electrolysis, carbon felt was treated with ultrasound in ethanol for 10 minutes to degreasing, degassing and improving the wettability of surface. After electroplating, the samples were rinsed in warm (60 °C) and room (25 0 C) distillate and dried at 100 °C for 1 hour.

Weighing of samples (carbon felt + copper wire) for gravimetric analysis was performed before ultrasonic treatment (m0) and after drying (mi). The thickness of the synthesized coatings was measured using a scanning electron microscope TESCAN SEM with the VEGA 3 SBH microanalyzer.

In the second part of the study, nickel electrodeposition was performed with electrochemical pre-treatment of carbon felt in solutions of 1M Na2SO4 and 1 M KOH for 10 min at a current density of 10 mA cm-2. Next, the samples were rinsed in distillate and nickel electrodeposition was performed at a current density of 10 mA cm-2 for 1 hour. After that, the samples were rinsed in warm and room distillate and dried for 1 hour at 100 °C. The synthesized coatings were labeled Ni@CF-Na2SO4 and Ni@CF-KOH, respectively.

2.2.2. Voltammetric techniques. Cyclic voltammetry (CVA) was executed in a three-electrode cell using a potentiostat-galvanostat ELINS P-45X equipped with a frequency response analyzer module FRA-24M. During the measurements, carbon felt samples with dimensions of 2.75 x 0.5 x 0.5 (cm) (Sgeo = 6 cm2) were used, mounted on a JJ110 sample holder with a platinum plate as a collector. The platinum plate (S = 1 cm 2) and Ag/AgCl (E°g/AgCl = 0.194 V, 3 M KCl) were counter and reference electrodes, respectively. The obtained polarization curves were converted to the scale of the normal hydrogen electrode (NHE):

En HE — EAg/AgCl + EAg/AgCl- (!)

When measuring the capacitance of electric double layer, solutions of H2SO4, Na2SO4 and KOH with a concentration of 1 M were used as electrolytes. The mass of the sample was m — 70 mg. Previously, the sample was treated with ultrasound in ethanol for 10 minutes to degreasing, degassing and improving the wettability of surface. The solution was stirred using a magnetic stirrer. Measurements were performed from the open circuit potential (OCP) to the potential corresponding to overpotential of HER dEHER — -400 mV. Scanning rates (v) were 10, 20, 40, 60, 80, 100 mVs-1.

To study the redox reaction

[Fe(CN)6]4- - e ^ [Fe(CN)6f-

a solution of 0.005 M K4Fe(CN)6 + 0.1 M KCl was used as the electrolyte. The mass of the sample in this test was m — 78 mg. Measurements were carried out starting from the OCP to the anode side. The range of scanning potentials was from -200 to 800 mV vs Ag/AgCl. Scanning rates (v) were 10, 20,40, 60, 80, 100 mVs-1.

2.2.3. Characterization of pre-treated nickel coatings. The structure and morphology of the synthesized samples were studied using a scanning electron microscope TESCAN SEM with the VEGA 3 SBH microanalyzer (EDX-mapping). XRD analysis was performed on a Rigaku SmartLab III diffractometer (CuKa radiation, A — 0.15405 nm).

The electrocatalytic properties of Ni@CF-Na2SO4 and Ni@CF-KOH were evaluated by the CVA with the determination of Tafel slope, ECSA and the turnover frequency (TOF). The methodology and equipment used are similar to paragraph 2.2.2.

2.3. Calculation methods

2.3.1. The ECSA from Faraday's Law (AED). The calculation of the ECSA value from the thickness of electrodeposited nickel was carried out in two steps. Initially, a selection of values was collected from the coating thicknesses in the SEM images (Fig. A1, Appendix). The average number of values in the selection for each sample was 35. Further, the values were statistically processed to determine the absolute and relative (e) errors (probability P — 0.99). At the second step, the values obtained for the samples synthesized in the coulostatic mode were compared with each other, and for the samples electrodeposited in the galvanostatic mode, a graphical dependence of the coating thickness on the electrolysis time was constructed.

According to Faraday's law

m — qNi • Q • FE, (2)

pAw H — qNi • It • FE, (3)

H (t)— • FE • t, (4)

pAw

u t+\ + qNi •1 •FE ^

H (t) — n • t; n —---, (5)

pAw

where m is the mass of deposited nickel (g), qNi is the electrochemical equivalent of the nickel plating process (gA-1 h-1), Q is the amount of electricity (C), FE is the Faraday efficiency, pNi is the nickel density (gcm-3), Aw is the working surface area (cm2), H is the coating thickness (cm), I is the current (A), t is the electrolysis time (h). Hence the working surface Aw is as follows

q • I • fe

Aw — --, (6)

p • n

and the specific value of ECSA:

Aed = Sh ■ (7)

S geo

2.3.2. The ECSA from EDLC value (ADL). To calculate the EDLC, a graph was plotted in the coordinates Aj — f (v), where

Aj — lja + jc|, (8)

Aj is the arithmetic mean of the anode (ja) and cathode (jc) current densities (mA cm-2).

The EDLC (Cdi) was obtained from the slope of the approximation line. Further, the specific value of ECSA was calculated according to the expression

Adl — Cl, (9)

where C* — 0.08 mF cm 2 is the specific value of the capacitance on a porous, highly active and inhomogeneous surface.

2.3.3. The ECSA from Randles-Sevcik equation (ARS). To determine the specific value of ECSA during CVA in a solution of 0.005 M K4Fe(CN)6 + 0.1 M KCl, a graphical dependence jpa,geo — (v)°'5 was constructed, according to the Randles-Sevcik equation

jpageo = 2.69 X 105ArsCoXn1'5D°'5V°'5, (10)

jpageo = k • V °'5; k = 2.69 X 105 Ars Cox n1'5 D °'5, (11)

where jpageo is anodic peak current density (per geometric surface) (A-cm-2), 2.69 • 105 is the constant of the Randles-Sevcik equation (C-mole-1 -V-0'5), ARS is the electrochemical active surface area (cm2 per 1 cm2 geometric), Cox is the concentration of the oxidized form (mole-cm-3), n is the number of electrons involved in the reaction, D is the diffusion coefficient (cm2s-1), v is the scanning rate (V-s-1).

From the slope value (k) of the linear approximation, the specific value of ECSA was calculated by the following expression

k

ArS = 2.69 • 105 • Cox-n1-5-D°'5' (12)

3. Results and discussion

3.1. Evaluation of the ECSA from coating thickness by Faraday's law

Table 2 presents the results of gravimetric analysis for carbon felt samples and the average Faraday efficiency (FE) for the nickel electrodeposition process.

Table 2. Results of nickel electroplating

Sample Current, mA Time, h mo, mg mi, mg Am, mg A mteo, mg FE

Ni-30-0.5 30 0.5 106.3 121.8 15.5 16.4 0.94

Ni-30-1 30 1 104.4 134.5 30.1 32.9 0.92

Ni-30-2 30 2 108.0 172.0 64.0 65.7 0.97 FE — 0.95

Ni-15-2 15 2 106.6 137.6 31.0 32.9 0.94

Ni-60-0.5 60 0.5 102.7 134.8 32.1 32.9 0.98

It can be noted that the process proceeds with high efficiency (95 %), the remaining 5 % of electricity is spent to the hydrogen evolution reaction on the forming coating surface. Fig. 1 shows the results of measuring the thickness of the coating during coulostatic and galvanostatic electrodeposition of nickel on carbon felt.

In coulostatic electrolysis, the thickness of the nickel coating is H1 = 0.57 ± 0.02 microns (e = 3.5 %). When passing the same amount of electricity in galvanostatic mode, the nickel coating has a thickness of H2 = 0.54 ± 0.02 microns (e = 2.9 %). The working surface areas, in this case, are equal

q • Q • FE 1.095 • 0.03 • 0.95

Aw1 —

PNi • Hi 8.9 • 0.57 • 10-4

-r — 62.2 cm2, AA1 — ±(62.2 • 0.035) — ±2.2 cm2,

Fig. 1. Results of electroplating nickel in coulostatic (a) and galvanostatic (b) modes

Aw2 —

q • Q • FE 1.095 • 0.03 • 0.95

-r — 65.6 cm2, AA2 — ±(65.6 • 0.029) — ±1.9 cm2.

pNi • H2 8.9 • 0.54 • 10-

These values are overlapped in the region Aw — (64.05 ± 0.35) cm2, and the specific ECSA value of the carbon felt during electrodeposition is

64.05 2 2

— 21.35 cm2 per 1 cm2 geometric.

A — Aw Aed —

S

geo

3

3.2. Evaluation of the ECSA from double layer capacitance

Determination of the ECSA from the value of EDL capacitance is one of classical methods. Fig. 2(a) shows the linear approximations in the coordinates Aj — v. The slope coefficient of these lines is numerically equal to the capacitance of double layer. Using expression (9), ECSA values for carbon felt in acidic, neutral and alkaline environments were calculated. From the presented histograms (Fig. 2(b)), we can conclude about the advantage of alkaline solutions when using carbon felt as an electrode material. The ECSA value in alkaline, acidic and neutral media is 95, 34.3 and 26.8 cm2 per 1 cm2 geometric. Presumably, this result is caused by the greater lyophilicity of the carbon filament in an alkaline medium compared to acidic and neutral media.

Fig. 2. Capacitance plot (a) and values of ECSA (b) for carbon felt in different electrolyte

It is worth noting that when using an acid solution as an electrolyte on the CVA curves (Fig. A2, Appendix), an anode peak is observed at a potential of E — 0.23 V vs NHE, which corresponds to the oxidation of ethanol used in pre-preparation.

3.3. Evaluation of the ECSA from Randles-Sevcik equation

The results of processing CVA curves are shown in Fig. 3.

It can be seen from Fig. 3(a) that with increasing scanning rate, the process taking place on the electrode under study becomes less reversible or quasi-reversible. The probable cause may be diffusion control due to the porosity of the electrode, which makes it difficult for natural convection and the movement of reagents from the reaction zone into the bulk of the solution. This is confirmed by the slope of the linear approximation in log(jgeo) — log(v) coordinates

Fig. 3. CVA curves of [Fe(CN)6]4-/[Fe(CN)6]3- process (a) and the Randles-Sevcik plot (b)

(Fig. A3(a), Appendix) equal to m = 0.44. For reactions with diffusion limitations, it is as follows: m = 0.4 ± 0.1. From the linear approximation in the coordinates jpa,geo - v0 5, the slope was determined for further calculation of the surface area (Fig. 3(b)). The ECSA value from the Randles-Sevcik equation is equal to

0.011 2 2

Ars =-n^ = 51.6 cm2 per 1 cm2 geometric.

2.69 • 105 • 3.1 • 10-7-1L5-(6.5 • 10-6)

The calculation used the effective concentration of the oxidized form obtained by integrating the polarization curve at the lowest scanning rate in I-t coordinates (Fig. A3(b), Appendix). As a result of diffusion limitations, the pH of the near-electrode region shifts to a slightly-alkaline range, which gives a higher ECSA value. This value is consistent with the one previously obtained in work [1].

3.4. Comparison of the evaluation results and the ECSA value for carbon felt

Table 3 summarizes the evaluation of ECSA values obtained by various electrochemical techniques.

Table 3. The ECSA values of carbon felt by various electrochemical techniques

Technique ECSA, cm2 per 1 cm2 geometric

Faraday's law 21.4

34.3 (pH < 7)

Double layer capacitance 26.8 (pH « 7)

95.0 (pH > 7)

Randles-Sevcik equation 51.6

As can be seen, the specific value of ECSA for carbon felt lies in the range from 20 to 100 cm2 per 1 cm2 geometric surface. The ECSA significantly depends on the pH electrolyte used, and for acidic and neutral media, its range of values is 20 - 40 cm2 per 1 cm2 geometric surface. In an alkaline media, activation of the surface is observed [32], as a result of which the area increases at least 8-10 times. Comparing the results of ECSA with the average values of the specific surface area for carbon felt measured by the BET method, it can be determined that the ECSA in acidic and neutral media is 0.1 - 0.3 m2g-1, and in alkaline it is 0.4 - 0.8 m2g-1. This is 1.5 - 4 times lower than the average value Sbet = 1.2 m2g-1. This difference is, among other things, due to the low lyophilicity of carbon materials in aqueous electrolytes, as a result of which it is recommended to use surfactants, or to activate the surface by chemical and thermal methods.

3.5. Effect of carbon felt pre-treatment on electrocatalytic features of nanostructured nickel coatings

Figure 4(a,b) shows the SEM results with mapping. It can be seen from the SEM images that the morphology of coating with pre-treatment in an alkaline solution differs significantly from a similar process in a neutral medium. The Ni@CF-KOH coating is not smooth, unlike Ni@CF-Na2SO4, has a distinct defect in the form of dendrites on surface.

The results of XRD analysis (Fig. 4(c)) demonstrate the presence of a carbon felt phase at 20 - 30 degrees and peaks corresponding to ICSD card No. 426960, which describes the cubic modification of nickel. According to calculations, the observed broadening for the characteristic nickel peaks (Fig. 4(d)) indicates a decrease in the crystallite sizes from 26.5 to 15.1 nm.

The results of electrocatalytic measurements for synthesized coatings are shown in Fig. 5. When using as an electrolyte for the pre-treatment of 1 M KOH instead 1 M Na2 SO4, an improvement in the main electrocatalytic features of HER is observed.

Figure 5(a,b) demonstrates that the overpotential value shifts from -210 to -120 mV. At the same time, the absolute value of Tafel slope decreases. The value greater than 120 mVdec-1 indicates a reaction mechanism with the presence of diffusion limitations, which corresponds to the Volmer-Tafel model:

M* +H2O + e ^ MHads + OH- (Volmerstep)

2MHads ^ H2 +2M* (Tafel step)

The ECSA measurement shows a sharp increase in both the EDLC and the surface area value (Figs. 5(c) and A4). For Ni@CF-Na2SO4 and Ni@CF-KOH, the ECSA value is 133 and 700 cm2 per 1 cm2 geometric surface, respectively. There was also an improvement in the coating efficiency from the results of TOF-test (Fig. 5(d)).

It can be seen that the electrochemical activation of carbon felt in an alkaline solution allows obtaining a more catalytically active surface both in terms of the value of surface area and improving kinetic parameters. All this results in a synergistic effect expressed by an increase in the total number of molecules transformations per unit surface per unit time.

FIG. 4. SEM images (a, b) and XRD patterns (c, d) for synthesized samples

Fig. 5. LSV curves (a), Tafel plot (b), capacitance plot (c) and TOF result (d) for Ni@CF-Na2SO4 and Ni@CF-KOH

4. Conclusion

Based on the study results, it can be concluded that

(i) The evaluation of ECSA by various techniques gives convergent results within 1 order of magnitude when using electrolytes with the same pH value.

(ii) For a neutral medium, the ECSA of carbon felt can be assumed to be 20 - 30 cm2 per 1 cm2 geometric surface. For acidic and alkaline media, the ECSA value is 30 - 40 cm2 and 50 - 100 cm2 per 1 cm2 geometric surface, respectively.

(iii) When estimating the ECSA from the Randles-Sevcik equation, diffusion limitations are observed in the redox process. This shifts the pH of near-electrode layer to a slightly-alkaline range and overestimates the ECSA value of carbon felt.

(iv) It is shown that the electrochemical activation of carbon felt in neutral and alkaline solutions allows the synthesis of nanostructured nickel coatings with a crystallite size of 26 and 15 nm, respectively.

(v) Electrochemical pre-treatment of carbon substrate in 1 M KOH decreases the overpotential of HER and the Tafel slope for nickel coating to -120 mV and -135 mVdec-1.

Appendix

Ni-30-2 e)

\

Ni-30-0.5 c)

2 Mm

U-60-0.5

Fig. A1. SEM images of samples (a-e) and average coating thicknesses (e)

Fig. A2. CVA curves of carbon felt in 1 M H2SO4 (a,d), 1 M Na2SO4 (b,e), 1 M KOH (c,f)

Fig. A3. Logarithmic plot (a) and I — t graph of [Fe(CN)6]4-/[Fe(CN)6]3- process (v — 10 mVs-1)

Fig. A4. CVA curves of Ni@CF-Na2SO4 (a) and Ni@CF-KOH (b)

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Submitted 7 September 2023; revised 5 October 2023; accepted 7 October 2023

Information about the authors:

Dmitry S. Dmitriev - Ioffe Institute, 194021 Saint Petersburg, 26 Polytechnicheskaya street, Russian Federation; ORCID 0000-0002-7261-6694; elchemorg@gmail.com

Maksim I. Tenevich - Ioffe Institute, 194021 Saint Petersburg, 26 Polytechnicheskaya street, Russian Federation; ORCID 0000-0003-2003-0672; chwm420@gmail.com

Conflict of interest: the authors declare no conflict of interest.